Syringes intended for being used only for a single injection are already known. For example, French Patent Application No. 76 12 199 describes a combined ampoule and syringe, in which the piston carries, at its end placed inside the cylinder of the syringe, a joint which is separable from the piston, which is designed to remain in its final position at the bottom of the cylinder when the injection has been carried out. A retraction of the rod causes a separation between the rod and the joint of the piston. However, this syringe does not permit a liquid to be drawn in from an ampoule. On the other hand, in the construction as proposed, it is an easy matter to remove the joint or packing of the cylinder from the syringe by other means and to use the syringe again after having refilled it. German Patent Application No. 17 66 748 proposes a syringe of which the rod is provided with a locking device for preventing the return of the piston from a position of maximum insertion. Nevertheless, this arrangement, situated outside the cylinder, can be easily detached or made inoperative, so that this syringe does not give any guarantee for it not to be used again.
An effectively disposable syringe is described in French Patent Application No. 75 15 412. It comprises means associated with that part of the piston which is disposed inside the cylinder and permitting a single intake and ejection movement of the piston.
Now it is frequently necessary to effect the injection of a mixture of several different liquid substances, which mixture cannot be prepared in advance, but only at the moment of injection. This involves the necessity of having available a syringe which is capable of being filled in several steps, that is to say, a syringe of which the piston can carry out an indefinite number of reciprocatory movements during the filling of the cylinder before the operation of injection.